What will you do with your extra second?

The world’s official timekeepers have added a “leap second” to the last day of the year on Wednesday, to help match clocks to the Earth’s slowing spin on its axis, which takes place at ever-changing rates affected by tides and other factors.

The U.S. Naval Observatory, keeper of the Pentagon’s master clock, said it would add the extra second on Wednesday in coordination with the world’s atomic clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time, or UTC.

That corresponds to 6:59:59 p.m. EST (23:59:59 GMT), when an extra second will tick by — the 24th to be added to UTC since 1972, when the practice began.